The benefits of delivering drug-loaded prostheses to patients are becoming well known. Studies have shown the importance of delivering the correct drug dose density on coronary stents to prevent restenosis by application of drugs such as paclitaxel or rapamycin. Numerous processes have been proposed for the application of such a coating including: soaking or dipping the implantable device in a bath of liquid medication, soaking in an agitated bath, introducing heat and/or ultrasonic energy in conjunction with a medicated bath, and spraying the medication by way of pressurized nozzles.
Initially, such coatings were applied at the time of manufacture of the medical device. For various reasons, including the relatively short shelf life of some drugs compared to the time span from manufacture to implantation, a need has arisen for technologies that permit applying a coating to a medical device just prior to implantation of the device sometimes referred to as application at the “point of care.” Just in time delivery of properly dosed devices would address the shelf-life limitations of various medicinal coatings as well as provide custom treatment.
Many of the methods and devices intended to coat a device just prior to implantation, however, deposit the coating material onto any and all surfaces that are exposed to the coating. This may result in depositing coating material on surfaces for which the coating is unwanted or undesirable. Further, the coating may crack or break away when the implantable device is removed from the implantation apparatus. An example of this would be a stent deployed on a catheter balloon. As the balloon is inflated, and the stent is expanded into position, the coating may crack along the interface between the stent and the balloon. These cracks may lead to a breaking away of a portion of the coating from the stent itself. Similar problems can occur in cases where the coating technique fails to prevent inadvertent overlapping with the edges, i.e., the internal surfaces along the edges of various devices, e.g., struts of stents. This, in turn, may affect the medicinal effectiveness of the coating, and negatively affect the entire medical procedure.
It is known to use ink-jet technology to apply a liquid to selected portions of a surface. An ink-jet nozzle moves in three dimensions with respect to the device to be coated with the aid of a motion control system. The motion control system enables the ink-jet nozzle to move over the portions of the prosthesis to be coated. Alternatively, a real-time picture can be taken with a camera to determine the position of the ink-jet nozzle in relation to the prosthesis. Based upon the feedback of nozzle location, the ink-jet applicator can be controlled by activating the spray, moving the ink-jet nozzle, and/or moving the prosthesis to adjust to the pattern to better conform with the actual prosthesis.
For systems where the motion is preprogrammed, selected patterns may fail to accommodate inherent variability in the surface of the prosthesis. In one non-limiting embodiment, for example, a stent crimped around a balloon catheter will not be crimped the same as a next crimped stent such that successive crimped stents present the same surface each time. The crimping cannot be determined from the factory according to the manufacturer's specifications of the stent.